Electrostatic protection circuit

ABSTRACT

An electrostatic protection circuit includes a first power line and a second power line. The electrostatic protection circuit includes a trigger circuit connected between the first and second power lines and outputs a trigger signal in response to a fluctuation of a voltage difference between the first and second power lines. The electrostatic protection circuit further includes a shunt element that is controlled by the trigger signal, and includes a main current pathway connected between the first and second power lines. The electrostatic protection circuit further includes a control circuit that is connected between the first and second power lines and supplies a control signal for increasing the conductivity of the shunt element when the voltage difference between the first power line and the second power line exceeds a predetermined voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/040,748 filed on Feb. 10, 2016, which claims priority from JapanesePatent Application No. 2015-046362, filed Mar. 9, 2015, the entirecontents of each of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an electrostaticprotection circuit.

BACKGROUND

Various proposals for a protection circuit against ElectrostaticDischarge (ESD) have been made heretofore. ESD means a discharge from anelectrically charged body or machine to a semiconductor device and adischarge from an electrically charged semiconductor device to theground potential. When an ESD occurs in a semiconductor device, a largeamount of electric charge from the terminal flows into the semiconductordevice as a current and the electric charge generates a high voltageinside the semiconductor device, hence causing, for example, adielectric breakdown of the internal elements and subsequently a faultin the semiconductor device.

A representative example of the electrostatic protection circuit is anRC triggered (RCT) MOS circuit. The RCT MOS circuit uses a triggercircuit comprising a serial circuit including a resistor and a capacitorserially connected between power terminals. A voltage at the connectionpoint between the resistor and the capacitor is set as a trigger signalfor driving a shunt transistor for discharging the electrostatic surge.By turning on the shunt transistor, a discharge current of an ESD surgeflows in a power line and a high voltage may be applied to the internalcircuit also connected to the power line. It is desirable to avoidbreaking the internal circuit as a result of a high voltage beingapplied due to the discharge of the ESD surge.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an electrostatic protection circuit according to a firstembodiment.

FIG. 2 depicts an electrostatic protection circuit according to a secondembodiment.

FIG. 3 depicts an electrostatic protection circuit according to a thirdembodiment.

DETAILED DESCRIPTION

According to at least one embodiment, an electrostatic protectioncircuit capable of preventing a breakdown of an internal circuit isprovided.

In general, according to one embodiment, an electrostatic protectioncircuit includes a trigger circuit connected between a first power lineand a second power line. The trigger circuit is configured to output atrigger signal in response to a fluctuation or change in a voltagedifference between the first power line and the second power line, suchas occurs when an electrostatic discharge is supplied to a device. Ashunt element in the electrostatic protection circuit has a currentpathway between the first power line and the second power line. Aconductivity of the current pathway is controlled according to thetrigger signal. For example, the resistance in the current pathway canbe decreased when the voltage difference is at or above some level. Acontrol circuit in the electrostatic protection circuit is connectedbetween the first power line and the second power line. The controlcircuit is configured to supply a control signal to the shunt elementfor increasing the conductivity of the current pathway when the controlcircuit detects the voltage difference between the first power line andthe second power line exceeds a predetermined voltage. In someembodiments, the predetermined voltage can be set according to themaximum rated voltage of an internal circuit for which the electrostaticprotection circuit is intended to protect from ESD shocks.

An electrostatic protection circuit according to various embodimentswill be hereinafter described in detail with respect to variousdrawings. The scope of the present disclosure is not limited to theseexample embodiments, but also includes variations, combinations, andmodifications of the examples which would be apparent to those ofordinary skill in the art.

First Embodiment

FIG. 1 depicts an electrostatic protection circuit according to a firstembodiment. The electrostatic protection circuit according to the firstembodiment is provided in a semiconductor chip 10. The electrostaticprotection circuit according to the first embodiment includes a firstpower line 24 connected to a first power terminal 20. For example, apower voltage VCC of a high potential side of a power supply or the likeis applied to the first power terminal 20 in the normal state. Thecircuit further includes a second power line 26 connected to a secondpower terminal 22. For example, a ground potential VSS of a lowpotential side is applied to the second power terminal 22 in the normalstate.

The electrostatic protection circuit according to the first embodimentincludes a trigger circuit 30 that is connected to the first power line24 at a contact portion 100 and is connected to the second power line 26at a contact portion 101. The trigger circuit 30 is connected to thefirst power line 24 and the second power line 26 in the contact portion100 and the contact portion 101, for example, through vias (notspecifically illustrated) formed in an insulating film (not specificallyillustrated) provided in the semiconductor chip 10. In this context,“vias” are conductive elements used to make layer-to-layer electricalconnections. The first power line 24, the second power line 26, and asignal input/output line 23 are connected to other circuit elements in asimilar manner. The trigger circuit 30 outputs a trigger signal inresponse to a voltage difference generated between the first power line24 and the second power line 26.

The electrostatic protection circuit according to the embodimentincludes a shunt circuit 32 connected between the first power line 24and the second power line 26. The trigger signal from the triggercircuit 30 is supplied to the shunt circuit 32. The shunt circuit 32includes, for example, a shunt element (not specifically illustrated)formed by a MOS transistor with a source-drain path, that is the maincurrent path of the MOS transistor, connected between the first powerline 24 and the second power line 26. The shunt circuit is connected tothe first power line 24 at contact portion 110 and to the second powerline 26 at contact portion 111.

The electrostatic protection circuit according to the first embodimentincludes an overvoltage detection circuit 34 connected between the firstpower line 24 and the second power line 26. The overvoltage detectioncircuit 34 is connected to the first power line 24 at a contact portion120 and is connected to the second power line 26 at a contact portion121. When a voltage between the contact portion 120 of the first powerline 24 and the contact portion 121 of the second power line 26 exceedsa predetermined threshold voltage, the overvoltage detection circuit 34supplies a control signal to the shunt circuit 32. The control signalsupplied by the overvoltage detection circuit 34 increases the currentflowing in the shunt element of the shunt circuit 32 and reduces theshunt resistance value while enhancing the conductivity of the shuntelement.

An internal circuit 36 is connected to the first power line 24 at acontact portion 130 and is connected to the second power line 26 at acontact portion 131.

The internal circuit 36 receives an input signal through the signalinput/output line 23 connected to the input/output terminal 21 and asignal line 151, and outputs an output signal through the input/outputterminal 21.

The cathode of an ESD protection diode 40 is connected to the firstpower line 24 at a contact portion 140, and the anode is connected tothe signal input/output line 23 at a contact portion 150. Similarly, thecathode of an ESD protection diode 41 is connected to the signalinput/output line 23 at the contact portion 150, and the anode isconnected to the second power line 26 at a contact portion 141.

When a positive ESD surge with respect to the second power line 26(e.g., ground potential) is applied to the first power line 24, thetrigger circuit 30 supplies the trigger signal to the shunt circuit 32.In response to the trigger signal from the trigger circuit 30, the shuntcircuit 32 is turned on (forms a conductive pathway between the firstpower line 24 and the second power line 26). According to this, adischarge path of the ESD surge is formed between the first power line24 and the second power line 26. When a negative ESD surge with respectto the second power line 26 is applied to the first power line 24, adischarge path of the ESD surge is formed by the ESD protection diodes(40, 41).

When a positive ESD surge with respect to the second power line 26 isapplied to the input/output terminal 21, the trigger circuit 30connected between the first power line 24 and the second power line 26responds and supplies the trigger signal to the shunt circuit 32. Inresponse to the trigger signal, the shunt circuit 32 is turned on and adischarge path of the ESD surge is formed between the first power line24 and the second power line 26.

By turning on the shunt circuit 32, a discharge current of the ESD surgeflows in the first power line 24 and the second power line 26. Accordingto this discharge current, a voltage drop occurs in the respectiveresistors (27, 28) (hereinafter, referred to as wiring resistors orwiring resistances) of the respective power lines (24, 26). Theovervoltage detection circuit 34 is connected to the respective powerlines (24, 26) at the contact portions (120, 121) closer to theinput/output terminal 21 than the contact portions (110, 111) where theshunt circuit 32 is connected to the respective power lines (24, 26).According to this, a voltage rise amount caused by a voltage dropoccurring in the wiring resistances (27, 28) due to the dischargecurrent flowing in the power lines (24, 26) may be reflected in thedetection voltage of the overvoltage detection circuit 34. The wiringresistances can be considered to be uniform over the entire lengths ofthe first power line 24 and the second power line 26; however, forconvenience sake of the description, only the wiring resistance 27between the contact portion 110 and the contact portion 120 and thewiring resistance 28 between the contact portion 111 and the contactportion 121 are specifically illustrated.

For example, a voltage drop generated in the wiring resistance 27 by thedischarge current of the ESD surge discharged from the input/outputterminal 21 through the shunt circuit 32 increases the voltage in thecontact portions (130, 131) to which the internal circuit 36 isconnected, as compared to the voltage in the contact portion 110 towhich the shunt circuit 32 is connected. Similarly, since the voltage onthe side of the overvoltage detection circuit 34 rises, an increase inthe voltage applied to the internal circuit 36 by the shunt circuit 32discharging the ESD surge may be detected by the overvoltage detectioncircuit 34. The overvoltage detection circuit 34 detects an overvoltageapplied to the internal circuit 36 and the shunt circuit 32 iscontrolled to reduce the voltage between the first power line 24 and thesecond power line 26, hence to be able to avoid such a situation that anovervoltage is applied to the internal circuit 36. When the overvoltagedetection circuit 34 detects a voltage exceeding the threshold value,the conductivity of the shunt element of the shunt circuit 32 isenhanced to increase the current flow and the resistance value of theshunt element is reduced to lower the voltage difference between thepower lines (24, 26), hence to be able to lower the voltage applied tothe internal circuit 36. As the result, the internal circuit 36 may beprotected from the breakdown caused by the overvoltage.

According to the first embodiment, when the voltage between the firstpower line 24 and the second power line 26 exceeds a predeterminedthreshold voltage, a overvoltage detection circuit 34 outputs a controlsignal to increase the current amount flowing in the shunt elementforming the shunt circuit 32, hence to reduce the resistance value ofthe shunt element that is connected between the first power line 24 andthe second power line 26, and thus lower the voltage difference betweenthe power lines (24, 26). As the result, the voltage applied to theinternal circuit 36 may be lowered. According to this process, since thevoltage applied to the internal circuit 36 may be lowered, it ispossible to avoid a situation that an overvoltage is applied to theinternal circuit 36 during discharge of an ESD surge.

Second Embodiment

FIG. 2 is a view illustrating an electrostatic protection circuitaccording to a second embodiment. In the electrostatic protectioncircuit according to the second embodiment, a trigger circuit 30connected between the first power line 24 and the second power line 26includes a CR series circuit including a capacitor 300 and a resistor301 connected in series. A trigger signal is output from a commonconnection node 302 of the capacitor 300 and the resistor 301.

The trigger signal of the trigger circuit 30 is connected to a gate ofan NMOS transistor 320 in the shunt circuit 32 through a buffer circuit60. The buffer circuit 60 includes a two stage serial connection ofinverters (61, 62). The respective inverters (61, 62) are, for example,CMOS inverters. The trigger signal of the trigger circuit 30 iswave-shaped and is amplified by the buffer circuit 60 to enhance drivingability of the signal ultimately supplied to the gate of the NMOStransistor 320. According to this structure, the current flow capacityof the NMOS transistor 320 may be increased. As the result, dischargeability with respect to the ESD surge may be enhanced.

The source of the NMOS transistor 320 is connected to the second powerline 26 in the contact portion 111, and the drain is connected to thefirst power line 24 at the contact portion 110. One end of a resistor 50is connected to the gate of the NMOS transistor 320 and the other end ofthe resistor 50 is connected to the second power line 26 at a contactportion 51. The resistor 50 is used in order to set a bias point of thegate of the NMOS transistor 320.

The overvoltage detection circuit 34 according to the second embodimentis connected to the first power line 24 at the contact portion 120 andis connected to the second power line 26 at the contact portion 121. Theovervoltage detection circuit 34 includes a serial circuit of a resistor340 and a diode 341 connected in series between first power line 24 andsecond power line 26. The cathode of the diode 341 is connected to oneend of the resistor 340, and the anode is connected to the second powerline 26. In short, the diode 341 is disposed to be reversely biased bythe power voltage VCC supplied to the first power terminal 20 and theground potential VSS applied to the second power terminal 22 in thenormal state.

The overvoltage detection circuit 34 includes a PMOS transistor 343. Thegate of the PMOS transistor 343 is connected to the common connectionnode 342 of the resistor 340 and the diode 341. The source of the PMOStransistor 343 is connected to the first power line 24 at the contactportion 120 and the drain is connected to the gate of the NMOStransistor 320.

When a positive ESD surge with respect to the second power line 26 isapplied to the first power line 24, the trigger circuit 30 outputs atrigger signal. The trigger signal is supplied to the gate of the NMOStransistor 320 through the buffer circuit 60 to turn on the NMOStransistor 320. According to this process, a discharge path for the ESDsurge is formed between the first power line 24 and the second powerline 26. When a negative ESD surge with respect to the second power line26 is applied to the first power line 24, a discharge path for the ESDsurge through the ESD protection diodes (40, 41) is formed.

When a positive ESD surge with respect to the second power line 26 isapplied to the input/output terminal 21, the trigger circuit 30, turnson the NMOS transistor 320 of the shunt circuit 32. As the result, adischarge path for the ESD surge is formed between the first power line24 and the second power line 26. When the voltage between the contactportion 120 and the contact portion 121 exceeds the breakdown voltage ofthe diode 341 of the overvoltage detection circuit 34, the diode 341 isturned on. For example, when the voltage drop caused by the wiringresistance 27 of the first power line 24 is raised according to theincreased discharge current due to the ESD surge and the voltage betweenthe first power line 24 and the second power line 26 exceeds thebreakdown voltage of the diode 341 in the overvoltage detection circuit34, the diode 341 is turned on (forms a conductive pathway).

When the voltage drop generated in the resistor 340 by turning on thediode 341 exceeds the threshold voltage of the PMOS transistor 343, thePMOS transistor 343 is turned on. When the PMOS transistor 343 is turnedon, the potential of the gate of the NMOS transistor 320 rises.According to this process, the drain current of the NMOS transistor 320is increased, the resistance between the source and the drain of theNMOS transistor 320 is reduced and the voltage between the first powerline 24 and the second power line 26 is reduced. In short, by theresponse of the overvoltage detection circuit 34, the voltage betweenthe first power line 24 and the second power line 26 may be reduced,hence to avoid such a situation that the overvoltage is applied to theinternal circuit 36.

The threshold voltage to which the overvoltage detection circuit 34responds is set as a voltage lower than the withstand (breakdown)voltage of the internal circuit 36. This is to protect the internalcircuit 36 from the breakdown due to the application of the overvoltage.For example, the threshold voltage may be set as a voltage lower thanthe absolute maximum rated voltage of the internal circuit 36.

In the second embodiment, the threshold value to which the overvoltagedetection circuit 34 responds may be set according to the breakdownvoltage of the diode 341 connected to the resistor 340. The resistor 50connected between the gate of the NMOS transistor 320 and the secondpower line 26 serves to set the potential of the gate of the NMOStransistor 320 when the PMOS transistor 343 is turned on. In otherwords, when the PMOS transistor 343 is turned on, the voltage betweenthe first power line 24 and the second power line 26 is divided by theon-resistance of the PMOS transistor 343 and the resistor 50 and appliedto the gate of the NMOS transistor 320. Accordingly, by properly settingthe value of the resistor 50, the bias point of the NMOS transistor 320when turning on the PMOS transistor 343 may be adjusted. According tothis, it is possible to achieve a bias so that an excessive current doesnot flow in the NMOS transistor 320. Here, the diode 341 may be formedby a Zener diode, for example.

Third Embodiment

FIG. 3 is a view illustrating an electrostatic protection circuitaccording to a third embodiment. In the electrostatic protection circuitaccording to the third embodiment, the overvoltage detection circuit 34includes a serial circuit of a resistor 340 and three stages of diodes(350, 351, 352) connected between the first power line 24 and the secondpower line 26 in series. The three stages of the diodes (350, 351, 352)are connected in a forward bias state when the power voltage VCC isapplied to the first power terminal 20 and the ground potential VSS isapplied to the second power terminal 22 in the normal state.

In the third embodiment, for example, the three stages of the diodes(350, 351, 352) forming the overvoltage detection circuit 34 determinethe threshold voltage for operating the overvoltage detection circuit34. In other words, when the voltage between the first power line 24 andthe second power line 26 gets higher than the summed threshold voltagesof the three stages of the diodes (350, 351, 352), the three stages ofthe diodes (350, 351, 352) are turned on. When the voltage dropgenerated in the resistor 340 by turning on the three stages of thediodes (350, 351, 352) exceeds the threshold voltage of the PMOStransistor 343, the PMOS transistor 343 is turned on and the gatepotential of the NMOS transistor 320 is raised. By raising the gatepotential of the NMOS transistor 320, the conductivity of the NMOStransistor 320 enhanced, the drain current is thus controlled to beincreased. According to this, the resistance between the source and thedrain of the NMOS transistor 320 is reduced, the voltage between thefirst power line 24 and the second power line 26 is lowered, and thevoltage applied to the internal circuit 36 is lowered. According to thisprocess, it is possible to avoid such a situation that the overvoltageis applied to the internal circuit 36.

In the example embodiments, although the NMOS transistor 320 is used asthe shunt element, a PMOS transistor may be used instead. In this case,the polarity of the biases of other elements is properly changed.Although the example embodiments have been described using a MOStransistor as the shunt element, a bipolar transistor may be usedinstead. In the case of using the bipolar transistor, the main currentpath is an emitter-collector path and the control electrode is a baseelectrode rather than a gate electrode. In this case, in relation to thebias, an NPN transistor may be used instead of the NMOS transistor.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An electrostatic protection circuit, comprising:a trigger circuit connected between a first power line and a secondpower line and configured to output a trigger signal in response to avoltage fluctuation on the first power line and the second power line; ashunt element including a current pathway between the first power lineand the second power line, a conductivity of the current pathway beingcontrolled according to the trigger signal; and a control circuitconfigured to supply a control signal to the shunt element to increasethe conductivity of the current pathway when the control circuit detectsthe voltage fluction on the first power line and the second power lineexceeds a predetermined amount.
 2. The electrostatic protection circuitaccording to claim 1, wherein the control circuit is further configuredto supply the control signal to the shunt element so as to increase theconductivity of the current pathway in correspondence with an increasedvoltage difference above a predetermined voltage.
 3. The electrostaticprotection circuit according to claim 1, further comprising: an internalcircuit connected between the first power line and the second powerline, the internal circuit being connected to the first power line at afirst position and to the second power line at a second position,wherein the control circuit is connected to the first power line at athird position and to the second power line at a fourth position, theshunt element is connected to the first power line at a fifth positionand to the second power line at a sixth position, and the third positionis between the first and fifth positions along the first power line, andthe fourth position is between the second and sixth positions along thesecond power line.
 4. The electrostatic protection circuit according toclaim 1, wherein the shunt element comprises a n-channelmetal-oxide-semiconductor transistor.
 5. The electrostatic protectioncircuit according to claim 1, wherein the shunt element comprises abipolar junction transistor.
 6. The electrostatic protection circuitaccording to claim 1, wherein the trigger circuit comprises a capacitorand resistor connected in series between the first and second powerlines.
 7. The electrostatic protection circuit according to claim 1,further comprising: a buffer circuit connected between the triggercircuit and the shunt element and configured to amplify the triggersignal and output the amplified trigger signal to the shunt element. 8.The electrostatic protection circuit according to claim 1, furthercomprising: an input/output signal line connected to an input/outputterminal and an internal circuit connected between the first and secondpower lines; a first protection diode connected between the input/outputsignal line and the first power line; and a second protection diodeconnected between the input/output signal line and the second powerline.
 9. The electrostatic protection circuit according to claim 1,wherein the shunt element is an n-channel transistor, the controlcircuit includes a first diode and a first resistor connected in seriesbetween the first and second power lines, an anode of the first diodebeing connected to the second power line, a cathode of the first diodebeing connected to the first resistor, a p-channel transistor includedin the control circuit has a control electrode connected to the cathode,a first end of the p-channel transistor being connected to first powerline and the first resistor, a second end of the p-channel transistorbeing connected to a control electrode of the n-channel transistor, anda second resistor is connected between the control electrode of then-channel transistor and the second power line.
 10. The electrostaticprotection circuit according to claim 11, wherein the first diode is aZener diode.
 11. The electrostatic protection circuit according to claim1, wherein the shunt element is an n-channel transistor, the controlcircuit includes a first plurality of diodes and a first resistorconnected in series between the first and second power lines, the firstplurality of diodes being connected in series with each other anode tocathode, a cathode end of the first plurality of diodes being connectedto the second power line, an anode end of the first plurality of diodesbeing connected to the first resistor, a p-channel transistor includedin the control circuit has a control electrode connected to the anodeend, a first end of the p-channel transistor being connected to firstpower line and the first resistor, a second end of the p-channeltransistor being connected to a control electrode of the n-channeltransistor, and a second resistor is connected between the controlelectrode of the n-channel transistor and the second power line.
 12. Anelectrostatic protection circuit, comprising a trigger circuitconfigured to output a trigger signal in response to a fluctuation in avoltage difference between a first power line and a second power line; afirst metal-oxide-semiconductor (MOS) transistor having a gate to whichthe trigger signal is supplied, and a main current pathway that isconnected between the first power line and the second power line; and acontrol circuit including: a first resistor and a first diode that areconnected in series between the first power line and the second powerline, and a second metal-oxide-semiconductor (MOS) transistor having agate connected to a connection node between the first resistor and thefirst diode, and a main current pathway connected between a first end ofthe first resistor and the gate of the first MOS transistor, the firstend of the first resistor being connected to the first power line and asecond end of the first resistor being connected to the connection node.13. The electrostatic protection circuit according to claim 12, furthercomprising: an internal circuit connected between the first power lineand the second power line, the internal circuit being connected to thefirst power line at a first position and to the second power line at asecond position, wherein the first end of the first resistor isconnected to the first power line at a third position and the firstdiode is connected to the second power line at a fourth position, afirst end of the main current pathway of the first MOS transistor isconnected to the first power line at a fifth position and a second endof the main current pathway of the first MOS transistor is connected tothe second power line at a sixth position, and the third position isbetween the first and fifth positions along the first power line, andthe fourth position is between the second and sixth positions along thesecond power line.
 14. The electrostatic protection circuit according toclaim 13, wherein a breakdown voltage of the first diode is less than orequal to a maximum rated voltage of the internal circuit.
 15. Theelectrostatic protection circuit according to claim 12, furthercomprising: a second resistor connected between the gate of the firstMOS transistor and the second power line; and a buffer circuit connectedbetween the trigger circuit and the gate of the first MOS transistor,the buffer circuit configured to amplify the trigger signal output fromthe trigger circuit.
 16. An electrostatic protection circuit,comprising: a trigger circuit connected between a first power line and asecond power line and configured to output a trigger signal in responseto a fluctuation in a voltage difference between the first power lineand the second power line; a first metal-oxide-semiconductor (MOS)transistor having a gate to which the trigger signal is supplied, and amain current pathway that is connected between the first power line andthe second power line; and a control circuit including: a first resistorand a first plurality of diodes connected in series between the firstpower line and the second power line, and a secondmetal-oxide-semiconductor (MOS) transistor having a gate connected to aconnection node between the first resistor and the first plurality ofdiodes, the first plurality of diodes being connected in series witheach other, anode to cathode, between the connection node and the secondpower line, a cathode end of the first plurality of diodes beingconnected to the second power line, an anode end of the first pluralityof diodes being connected to connection node, and a main current pathwayof second MOS transistor being connected between a first end of thefirst resistor and the gate of the first MOS transistor, the first endof the first resistor being connected to the first power line and asecond end of the first resistor being connected to the connection node.17. The electrostatic protection circuit according to claim 16, furthercomprising: an internal circuit connected between the first power lineand the second power line, the internal circuit being connected to thefirst power line at a first position and to the second power line at asecond position, wherein the first end of the first resistor isconnected to the first power line at a third position and the cathodeend of the first plurality of diodes is connected to the second powerline at a fourth position, a first end of the main current pathway ofthe first MOS transistor is connected to the first power line at a fifthposition and a second end of the main current pathway of the first MOStransistor is connected to the second power line at a sixth position,and the third position is between the first and fifth positions alongthe first power line, and the fourth position is between the second andsixth positions along the second power line.
 18. The electrostaticprotection circuit according to claim 17, wherein a summed thresholdvoltage of diodes in the first plurality of diodes is less than or equalto a maximum rated voltage of the internal circuit.
 19. Theelectrostatic protection circuit according to claim 16, furthercomprising: a second resistor connected between the gate of the firstMOS transistor and the second power line; and a buffer circuit connectedbetween the trigger circuit and the gate of the first MOS transistor,the buffer circuit configured to amplify the trigger signal output fromthe trigger circuit.
 20. The electrostatic protection circuit accordingto claim 16, wherein the first MOS transistor is a n-channel transistorand the second MOS transistor is a p-channel transistor.